1. Field of the Invention
The present invention relates to a screen printing apparatus and, more specifically, to an apparatus using a screen printing method suitable for filling minute through-holes with conductive paste or the like, which holes are made, for example, in circuit boards such as unfired ceramic boards called green sheets.
2. Description of the Related Art
As is well known, a multi-layered ceramic board for high-density mounting of semiconductor parts is obtained by pressing and firing a number of accurately stacked green sheets having electronic circuit patterns on their surfaces. The stacked green sheets are electrically connected by means of conductive paste with which through-holes provided in these green sheets are filled.
A green sheet is a flexible ceramic board having a thickness of approximately 0.2 mm. It has a large number of through-holes of approximately 0.2 mm extending from one side of the sheet to the other. In forming a multi-layered ceramic board, these through-holes are filled with conductive paste. The filling operation has conventionally been conducted by screen printing.
Japanese Unexamined Patent Publication No. 63-284893 discloses a conductive paste filling method, according to which a filter consisting of a highly permeable and dustproof plastic sheet is placed on a stage (a bearer) made of a porous material. A green sheet is placed on this filter and fixed in position by suction which is maintained during the hole-filling printing process, whereby the through-holes can be reliably filled with conductive paste. That part of the conductive paste which has passed through the through-holes is absorbed by the filter, thereby preventing adhesion of the conductive paste to the porous stage. Thus, a satisfactory attraction by suction can be maintained solely by changing the filter and the trouble of cleaning the stage can be avoided.
A problem with the conductive paste filling method disclosed in Japanese Unexamined Patent Publication No. 63-284893, referred to above, is that the conductive paste adheres to the plastic sheet (the filter) each time the through-holes of one green sheet are filled with the conductive paste. Thus, it is necessary to replace the plastic sheet with a new one each time the green sheet printing process is completed for one green sheet.
If a plurality of plastic sheets are supplied to the stage when a plastic sheet is replaced, the sucking capacity of the stage is remarkably lowered, thereby making it impossible to satisfactorily fill the green-sheet through-holes with paste. Thus, only one plastic sheet (filter) should be supplied to the stage at a time.
The above-mentioned Japanese Patent Publication discloses no specific method of replacing the plastic sheet. Manually replacement is difficult and time-consuming since the plastic sheet has very low bending strength due to its porosity and thinness and consequently is easily broken.